Most of the world’s time zones are whole hours from GMT (now UTC). India is UTC+5:30. The half hour comes from geography.
Fifteen degrees per hour
The Earth turns 360° in 24 hours: 15° every hour. A place 15° east of Greenwich sees the Sun an hour earlier. India stretches from about 68° E to 97° E. Choosing one meridian near the middle, 82.5° East, gives 82.5 ÷ 15 = 5.5 hours.
The Mirzapur line
The 82.5° E meridian passes near Mirzapur in Uttar Pradesh. A clock tower at Mirzapur is often said to mark the reference longitude for IST.
Before IST
In the 19th century, cities kept their own local time. Railways needed something simpler, so “Madras time” became widely used across the network. In 1906 the British government introduced a standard time of UTC+5:30, but Calcutta time and Bombay time stayed in local use for decades — Calcutta time until 1948 and Bombay time until 1955.
One zone for a wide country
Because India is about 2,900 km wide, the Sun rises nearly two hours earlier in Arunachal Pradesh than in Gujarat. There have been proposals for a second time zone for the North-East; tea gardens in Assam have long kept their own “chaibagan time”, an hour ahead of IST.
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